The workscope of this project renewal is the same as outlined in previous renewal applications with the following exceptions: We will obtain and test approximately ten fresh human rabies virus isolates and five fresh isolates each from dog, fox, skunk, raccoon, and bat; Human diploid cells shall be used for testing, in vitro, interferon sensitivity of these isolates; and The test will involve pretreatment of cells with interferon and placebo and subsequent challenge with the various strains of rabies viruses and control virus. Relative interferon sensitivity of the rabies viruses should be established to indicate variation due to strain difference.